Semiconductive electron multiplier



May 18, 1965 F. A. WHITE ETAL I SEMICONDUCTIVE ELECTRON MULTIPLIER Filed Nov. 3. 1960 FIGI I I I I I IQ I M I I A TTORNE Y 3,184,633 SEMICONDUCTIVE ELECTRON MULTIPLIER Frederick A. White and William D. Davis, Schenectady,

N .Y., assignors to. General Electric Company, a corporation of New York Filed Nov. 3, 1960, Ser. No. 67,002

' 3 Claims. (Cl. 31512) This invention relates to electron multipliers and more particularly to a magnetic electron multiplier in which a slab of semi-conductive material is used for the secondary emissive material.

In the past, most magnetic electron multipliers have had limited application due to the multiplicity of dynodes and resultant complexities and relatively bulky size. In addition, proper magnetic focusing has been exceedingly I difiicult to obtain and as a result, it has been virtually im- 7 The thin film in this type of electron multiplier has allowed only moderate potential gradients to be applied to'it and it is difiicult lto' acliievea uniform electrical gradient of desired stability; In 'addition, these thin films are frequently subject to burn out from excessive voltage gradicuts or from"envii'onme u tal disturbances.

It is, accotdinglyfan object of this invention to provide an improved and simplifiedmagnetieelectron multiplier.

It is a further object of"this* inventioh to provide'an improved single surface magnetic electron multiplier.

It is a further object of this invention to provide a single surface electron multiplier, the surface of which can be subjected to a high potential gradient.

It is a further object of this invention to provide a single surface electron multiplier, the surface of which is not subject to burn out.

It is a further object of this invention to provide an electron multiplier useful as an ion detector which will measure beam currents of exceedingly small magnitude.

It' is a further object of this invention to provide an electron multiplier which has a negligible background noise.

It is a still further object of this invention to provide an electron multiplier which is compact in size and of rugged assembly.

In accordance with one aspect of the invention, two

semiconductive slabs are mounted in a spaced parallel relationship and a longitudinal voltage gradient is provided along each of the semiconductive slabs. A biasing field is provided between the two slabs and a magnetic field is provided which is transverse to the electric fields caused by the previously mentioned voltage gradient and biasing field. An initial electron disturbance, such as by ion bombardment, is caused at the relative negative potential end of one of the semiconductive slabs. The elec- Q trons disturbed from the end of the slab will leave the slab and, under the influence of the electric fields and the magnetic field will follow a cycloidic path and will strike the semiconductive slab from whence it came a spaced distance along the semiconductive slab. This electron strikage will cause secondary emission of a number of electrons fromthe semiconductive slabs and each of the secondarily emitted electrons will also follow a cycloidic path and re-strike the semiconductive slab. This series of cycloidic paths will continue along the length of the semiconductive slab, with each of the succeeding cycloidic ice Patented May 18, 1965 loops having a greater number of electrons than the ductor slab to collect the electrons in the final cycloidic loop. This collector electrode may then be connected to any suitable utilization device, such as an amplifier or a pulse counter.

For a better understanding of the invention, reference may be had to the accompanying drawings in which:

FIG. 1 is a schematic illustration of an electron multiplier embodying the subject invention.

FIG. 2 is a view of a portion of the electron multiplier of FIG. 1 being used as a photon or light detector.

FIG. 3 is a view of a portion of an electron multiplier utilizing the subject invention and being used as an ion detector in conjunction with a mass spectrometer.

FIG. 4 shows one embodiment of the subject invention being used to detect the velocity of a charged particle.

FIG. 5 shows an electron multiplier embodying the subject invention and being mounted in a block which also serves as a heat reservoir for the device.

FIG. 1 shows a schematic representation of an electron multiplier in which two slabs of semiconductive material 1 ram 2*are mounted in a spaced, generally parallel relason; A longitudinal voltage gradient is" provided along e'achbf the slabs "1" and 2 by the voltage source 3. A biasing field between the two slabs 1 and 2 is provided by the voltage sources 4 and 5. 'Ihe electric fild 'r'esult ing from these voltage sources at an'y point on the slab 1 is represented by the vector E. 'Arnag'netic field'is provided from any suitable sourcetnot shown) which is transverse'to the voltage gradient in the semiconductive slabs andthe electric fieldbet'ween the slabs causedby the biasing sources 4 andfS. In the embodiment shown in FIG. 1, the magnetic lines of flux would pass into a sheet of paper. The entire assembly may be maintained in a vacuum by a suitable envelope 6.

The electron multiplier operates as followsrEnergy 7 from any suitable source impinges upon the relative negative potential end of the semiconductor slab 1. This energy may be in the form of charged particles such as ions or electrons or may be radiant energy such as light, ultra violet light, gamma rays, or any other form of energy which, when striking the semiconductor slab 1, will cause at least one electron to be emitted therefrom. These electrons, upon entering the electric field as represented by the vector IE, will be accelerated and, as they attain a velocity, will be deflected by the transverse magnetic field. The electron path 8 will follow a cycloidic loop whose base lies along the line 9, which is normal to the vector B. When an electron following this path restrikes the slab 1 secondary emission will occur from the slab and a plurality of electrons will then traverse another cycloidic loop in a similar manner. With each succeeding loop, the number of electrons in the path will increase. After the final loop, the electrons are collected by the electron collector 10 which is connected to ground, I through the resistor 11. An output signal will then appear across this resistor 11 and this signal may be applied to any desired utilization device 12, which may be an amplifier, a pulse counter, or any other desired utilization device.

The output pulse on the resistor 11 for each initial excitation of the slab 1 will be approximately K(X) where K is the average number of electrons ejected due to the primary excitation, X is the average number of secondary electrons caused to be emitted by each striking electron and n is the effective number of stages or loops which make up the electron path 8. It is thus seen that the electron multiplier is relatively insensitive to variations in the magnetic field. This is because while an increase in magnetic field will increase the number of loops in the eleceither N type or P type semiconductive materials.

may be made from gold doped silicon, cadmium sulphide,

tron path, it will also decrease the potential gradient through which each loop passes and will thus decrease the number of secondarily emitted electrons in the following loop or stage. Conversely, while a decrease in magnetic field strength will decrease the number of loops in the path 8, it will also increase the potential gradient to which each loop passes and will increase the number of second'arily emitted electrons in each loop. Thus, the device shows a relatively broad maximum gain for a varying magnetic field strength. 1

The semiconductive slabs 1 and 2 may be made from They interrnetallic compounds such as gallium arsenide or any other suitable semiconductive material. The resistivity of the semiconductive material may be increased by cooling the assembly, if desired. This increased resistivity f electron bombardment, but instead establishes the electric field between the slabs l and 2. Thus, the slab 2 must only be made of 'a material suflicient to support a voltage gradient similar to that in the slab 1. However, in the preferred embodiment of the invention the slab 2 is made of-thesame material as is the semiconductive slab 1,

thus assuring the same voltage gradient in each slab.

It has been'found that a gain in the order of 10 can be obtained from a device of the type described in which thelength of the semiconductor slabs is about two centimeters, the spacing between the slabs about one millimeter, the'voltage gradient from the source 3 is about 3,000 volts, the bias voltage from the sources 4 and 5 is about 400 volts and the magnetic field is about 1,000 gauss. Thesevalues were obtained while using gold doped silicon for thesemiconductor slabs and when the assembly was cooled with Dry ,Ice to a temperature of about 80 C.

, FIG. 2 shows a portion of an electron multiplier such as has been previously described being used as a light detector or photo tube. Suitable focusing means 16 are provided to direct and focus incident light onto a photo 1 cathode 15, which may be made of cesium antimony or any'other suitable photo emissive material. When the electrical and magnetic fields, the electrons follow a cy- 1cloidal path 8 andstrike the semiconductive slab 1, where upon secondary emission occurs. The operation of the device is then similar to that previously described.

' FIG. 3 shows an electron multiplier according to the subject invention being used as an ion detector in con- -junction with a mass spectrometer. A mass spectrometer 20 is generally shown including a curved portion of constant radius 21 which is joined to the electron multiplier by a member 23 that'includes an opening 24 therein.

The mass spectrometer 20, the arcuate region 21 and the electron multiplier are maintained in a common vacuum. Ions are accelerated by the mass spectrometer and a magnetic field (not shown) causes these ions to follow the arcuate path 22 and, if these ions are of proper predetermined mass, they will pass through the opening 24 and strike one of the ends of the semiconductive slab 1 of the electron multiplier. The ions striking the semiconductive slab 1 will cause electrons to be emitted therefrom and the electron multiplier will then operate in the manner hithertofore described. The electrical connections and magnetic field for the electron multiplier in this figure are not shown, but they will be made in a manner similar to that previously described. a

' FIG.4 shows one embodiment of the invention being used to measure the velocity of a high energy particle.

I Since velocity is a vectoral quality, it has both magnitude ki i I and direction andtheembodiment shown in FlG.'4.will determine both the direction and the magnitude of the velocity of a charged particle. A plurality of semiconductive slabs 31 through 38 are mounted in spaced, parallel relation. The potentiometer 40 provides a longitudinal voltage field gradient in each of the SCHllCOIldUCa.

tive slabs 31 through 38. The potentiometer 41 provides a biasing field potential between each ofthe adjacent ones of the semiconductive slabs 31 through .,38. A magnetic field (not shown) is provided transverse to the electrical fields and is in a direction into the paper at FIG. 4.

The arrow 39 represents the path of a charged particle having suificient velocity to penetrate to the semi 'become increasingly larger, as is shown by pulses 52,

through 56, since the electron paths associated with the semiconductive slabs 32 through 36 became increasingly longer, thus 'each surface showing a gain greater than the preceding surface.

photo cathode 15 has light impinging thereupon, electrons are emitted therefrom and, under the influence of the 0bviously, a particle of a given mass having a greater magnitude of velocity will penetrate a greater number of semiconductive slabs and thus the number of pulses which appear at the collector electrodes is indicative of the magnitude of the velocity of the particle striking the unit. Also, the ditference in the magnitude of the succeeding pulses 52 through 56 is a measure of the angle 0, since for greater angle 0, the total path length on succeeding stages will be longer and the change in gain from progressive stages will be greater. 1

FIG. 5 shows a mounting arrangement for an electron multiplier according to' the subject invention in which the spacings between the semiconductive slabs is accurately determined and which also removes any-heat which may be developed in the electron multiplier. ducti-ve slabs l and 2 are supported at their collector ends in a block 60. Copper or any other suitable material of good heat conductivity may be used for the block 60. The semiconductive slabs 1 and 2 are separated from the block 60 by sheets 61 and 62, thesesheets being of a material which is both electrically insulative and heat conductive, such as sapphire orruby. Again, the assembly may be maintained in a suitable vacuum and suitable electrical and, magnetic connections (not shown) similar to those previously discussed are provided.

While the invention is thus described and several applications of the invention are shown, the invention is obviously not limited to only these shown embodiments or applications. Numerous embodiments and changes will be obvious to those skilled in the art without departing from the spirit and scope of the invention. For example, thev materials and magnitudes of values given are for illustrative purposes only and the invention is obviously not limited to only these materials or values. The invention is therefore to be limited only by the scope of the appended claims.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In an electron multiplier having two spaced parallel electrode, means for establishing a biasing field between said electrodes, means for establishing a longitudinal potential gradient along each of said electrodes whereby each of said electrodes has a relative positive potential and a relative negative potential end, means for establisha The semiconcent the relative positive potential end of said electrodes, the improvement wherein at least said one of said electrodes consists entirely of a self-supporting slab of homogeneous semiconductor material having a length of about two centimeters and a resistivity of the order of ohm centimeters at a temperature of about 80 C., and said electrodes have a spacing of about one millimeter.

y 2. An electron multiplier according to claim 1 in which said one electrode isformed from high resistance golddoped silicon semiconductive material.

3. In a device for determining the velocity and direction of travel of a charged particle, an assembly comprising a plurality of spaced parallel slabs of secondary electron emissive semiconductor material having a spacing of about one millimeter and adapted to be disposed in the path of a charged particle, means for establishing a longitudinal potential gradient along each of said slabs of semiconductor material, each of said slabs thereby having a relative positive potential end and a relative negative potential end, means for establishing a biasing electric field between each pair of adjacent slabs, means for establishing a magnetic field transverse to said biasing fields and said longitudinal potential gradients whereby secondary electrons emitted from each of said slabs follows a cycloidal path therealong, and collector electrode means References Cited by the Examiner UNITED STATES PATENTS 2,715,684 8/55 Schwarz 250--71 2,841,729 7/58 Wiley 315--12 X 2,842,706 7/58 Dobischek et al. 313-347 X 2,983,845 5/61 Damoth et al. 131l03 X OTHER REFERENCES Soviet Phys. JEJP, 6, 619-20 (1958), translation.

GEORGE N. WESTBY, Primary Examiner.

ARTHUR GAUSS, Examiner. 

1. IN AN ELECTRON MULTIPLIER HAVING TWO SPACED PARALLEL ELECTRODE, MEANS FOR ESTABLISHING A BIASING FIELD BETWEEN SAID ELECTRODES, MEANS FOR ESTABLISHING A LONGITUDINAL POTENTIAL GRADIENT ALONG EACH OF SAID ELECTRODES WHEREBY EACH OF SAID ELECTRODES HAS A RELATIVE POSITIVE POTENTIAL AND A RELATIVE NEGATIVE POTENTIAL END, MEANS FOR ESTABLISHING A MAGNETIC FIELD TRANSVERSE TO SAID BIASING FIELD AND SAID LONGITUDIANL POTENTIAL GRADIENT, MEANS FOR CUASING AN ELECTRON EMISSION AT THE RELATIVE NEGATIVE POTENTIAL END OF ONE OF SAID ELECTRODES, AND AN ELECTRON COLLECTOR ADJACENT THE RELATIVE POSITIVE POTENTIAL END OF SAID ELECTRODES, THE IMPROVEMENT WHEREIN AT LEAST SAID ONE OF SAID ELECTRODES CONSISTS ENTIRELY OF A SELF-SUPPORTING SLAB OF HOMOGENEOUS SEMICONDUCTOR MATERIAL HAVING A LENGTH OF ABOUT TWO CENTIMETERS AND A RESISTIVITY OF THE ORDER OF 109 OHM CENTIMETERS AT A TEMPERATURE OF ABOUT -80* C., AND SAID ELECTRODES HAVE A SPACING OF ABOUT ONE MILLIMETER. 